An optical fiber 110 (FIG. 1) transmits an electromagnetic radiation 112, maintained in its optical core 114 due to a difference between the refractive index nc of the optical core 114 and the refractive index ng of the optical cladding 116, with a low attenuation, on the order of 0.20 dB/km, for a transmission having a wavelength of 1.55 μm.
Furthermore, a mechanical coating 118 surrounds the optical fiber 110 so as to make possible its handling without causing the latter to become fragile. Classically, this coating 118 is made of polyacrylate or polyimide.
The refractive index nc of the optical core 114 may undergo a longitudinal modulation in the optical core locally, according to a spatial period Λ or “pitch,” so that the optical fiber would reflect the radiation being propagated at a given wavelength λB. This local longitudinal modulation of the index constitutes a Bragg grating and the wavelength λB reflected is called the characteristic Bragg wavelength of the grating.
This wavelength λB may be predetermined by means of the Bragg equation which is written, in the first order:λB=2Λne(T, λ, [ε3×3])  (1)                where Λ is the characteristic pitch of the Bragg grating inscribed in the optical fiber, ne is the effective index of the guided dominant mode of the optical fiber, T is the temperature of the optical fiber in the grating, λ is the wavelength of the electromagnetic radiation and [ε3×3] is the 3×3 tensor of the Green-Lagrange deformations of the fiber.        
This [ε3×3] tensor of the Green-Lagrange deformations of the fiber is dependent on the local variations of the dimensions of the fiber, such as its length. These dimensions may vary depending, for example, on the hydrostatic pressure being applied to the section of optical fiber carrying the Bragg grating.
Consequently, it appears that the Bragg wavelength λB of a Bragg grating is dependent upon physical, mechanical and/or thermal parameters, having an effect on this grating.
Therefore, an optical fiber equipped with at least one Bragg grating may be used to measure physical parameters, for example, when these physical parameters have an effect on the length Lfib of the optical fiber at the level of a Bragg grating, such that a variation of this parameter leads to a change in the characteristic wavelength λB of the Bragg grating.
It should be specified that the phrase “deformation of the optical fiber” includes mechanical deformations, for example, deformations generated by a mechanical action such as an elongation force exerted on the fiber, and thermal deformations generated by a variation in temperature. For example, a variation in temperature may generate a variation in the effective index of the fiber. In fact, the temperature T to which a Bragg grating is subjected changes its characteristic wavelength λB.
Moreover, it is known that the determination of a deformation measured by a Bragg grating is optimal when its deformation remains homogeneous, i.e., when the grating passes from a pitch Λ at rest to a pitch Λm under the effect of a variation in the variable to be measured. An absence of deformation gradient along the grating guarantees such a homogeneity.
The variation of a parameter is measured by means of a light beam sent in the optical fiber from one of its ends, this beam comprising at least the wavelength λB of a Bragg grating inscribed in this optical fiber, as well as the Bragg wavelengths of this grating when it is subjected to variations in the measured physical parameter.
More precisely, the characteristic wavelength λB of the Bragg grating has a variation ΔλB when, for example, a variation ΔT in its temperature, ΔLfib in the length of the optical fiber and/or ΔP in the hydrostatic pressure occurs in the Bragg grating reflecting this wavelength λB.
When the fiber is deformed uniaxially along its axis of optical propagation, such an equation can be written by differentiating equation (1) in relation to the temperature T, the length Lfib of the optical fiber comprising the Bragg grating and the hydrostatic pressure P surrounding this optical fiber at the level of the Bragg grating. Thus, we obtain:Δ′λB/λB=a′ΔT+b′Δεfib+c′ΔP  (2)                where a′, b′ and c′ are, in a first approximation, constants peculiar to the nature of the optical fiber in question and Δ′λB is the variation in the characteristic wavelength λB of the Bragg grating, Δεfib is the variation in longitudinal mechanical deformation of the fiber, equal, in the first order, to the quotient ΔLfib/Lfib of the variation of mechanical origin ΔLfib of the length Lfib of the optical fiber.        
A measuring device is generally designed so that only the variable to be measured acts on the signal Δ′λB/λB effectively measured. For this purpose, it uses a test body 123 (FIG. 1b), on which is fixed, by means of two fixing points 121 and 125, the section of fiber 110 having a length Lfib, in which is inscribed at least one Bragg grating 124.
In this case, equation (2) is written as follows:ΔλB/λB=aΔT+bΔεce+cΔP  (3)                where a, b and c are constants depending on a′, b′ and c′, respectively, taking into account the geometry of the test body 123 and its thermomechanical characteristics.        
Moreover, Δεce represents the variation in mechanical deformation of the test body 123, which is equal, in the first order, to the quotient ΔLce/Lce of the variation of mechanical origin ΔLcein the length Lce of the test body.
By disregarding the effect of the pressure (c ΔP), this equation (3) makes it possible, starting from the measurement of the variations ΔλB in the wavelength reflected by the Bragg grating 124 of the optical fiber 110, to measure a deformation due to:                a variation in the temperature to which the Bragg grating 124 is subjected, and/or        a variation in the deformation ΔLce/Lce between the anchoring points 121 and 125 of the section of the optical fiber 110 carrying the Bragg grating 124.        
Such a measurement of variation in deformation ΔLce/Lce of the test body 123 can be used to measure a variation in force ΔF being exerted on this test body 123. In fact, it is possible, knowing the thermomechanical properties and the geometry of the test body, to establish a correspondence between the value of this variation in force ΔF and a variation ΔLce/Lce in the uniaxial deformation of any fiber aligned between the two anchoring points 121 and 125 of this test body.
In this example, the Bragg grating 124 optical fiber 110 is preloaded under tension between the two anchoring points 121 and 125 of the test body 123.
Thus, when an action is exerted on this test body 123, the latter is caused to deform, leading to a variation ΔLce in the distance between the two anchoring points 121 and 125, which can be measured by means of the variation ΔλB in the wavelength reflected by the Bragg grating 124 optical fiber 110.
In other words, the variations ΔLce in the length Lce of the test body are measured by the variations ΔλB in the Bragg wavelength λB reflected by the Bragg grating 124 inscribed in the optical fiber 110.
A measuring device equipped with a Bragg grating optical fiber has many advantages. For example, it makes it possible to put the spectral analysis system in charge of the measurement of the Bragg wavelength at a distance from the measurement point due to the low spectral attenuation of the optical fiber with respect to the radiation transmitted.
Such a distance is advantageous when, for example, the measurement is carried out in an environment that is hostile (elevated temperature and humidity) or not readily accessible for the signal processing means.
Other advantages lie in the fact that the optical fiber is insensitive to external electromagnetic interferences or that it behaves linearly in deformations, that it makes it possible to obtain a good resolution, and that it is insensitive to the aging of the end components (for example, laser or connection sources), the measurement principle being based on a spectral measurement, i.e., the characteristic Bragg wavelength of the grating.